In ultra-high rate optical communications, in order to resolve an insufficient optical signal to noise ratio (OSNR) and linear distortion such as wavelength dispersion, coherent detection and digital signal processing using analog-to-digital converters (ADCs) are becoming mainstream in place of conventional direct demodulation. Because of ultra-high data rates, oversampling of analog-to-digital conversion may not be affordable due to hardware limitation; therefore, it is demanded to sample data at the most suitable timing to satisfy a required signal quality. A method for optimizing phase compensation is proposed, in which method sampling is carried out at a frequency twice the symbol frequency and phase offset from the optimum sampling point is observed using a Gardner's phase detector (see, for example, Japanese Laid-open Patent Publication 2011-9956).
FIG. 1A is a schematic diagram of a known structure of a digital coherent receiver 1000. Signal light from an optical transmission path and local oscillator light are detected and the detection result is converted into an electric signal which is referred to as an O/E converted signal. The O/E converted signal is input to a digital converter 1150. The digital converter 1150 performs digital sampling on the O/E converted analog signal synchronized with a clock signal generated by a frequency variable oscillator 1140. The digitally sampled signal is subjected to signal processing at a digital signal processor 1160. In the digital signal processor 1160, waveform distortion is compensated for at a waveform distortion compensator 1161; digital phase compensation is carried out at a phase controller 1162; and adaptive equalization of distortion (i.e. compensation for waveform distortion) and demodulation are carried out at an adaptive equalization type demodulator 1163.
The distortion compensated input signal further undergoes phase compensation at a phase adjustor (PHA) 1511 of the phase controller 1162 and is supplied to a phase detector (PD) 1512, as well as to the adaptive equalization type demodulator 1163. The PD 1512 detects a phase shift from the optimum sampling point based upon an output of the phase adjustor 1511. The detected phase signal is fed back to the phase adjustor 1511 through a first digital loop filter (DLF) 1513 and also to a frequency variable oscillator 1140 through the first DLF 1513 and a second DLF 1514.
Influence of high-rate phase fluctuation (jitter) and fluctuation of local oscillator light are removed by feeding the phase signal back to the phase adjustor 1511, which includes finite impulse response (FIR) filters. Low-rate fluctuation such as wander is removed by feeding the phase signal back to the frequency variable oscillator 1140.
When a Gardner's phase detector is employed in the phase detector 1512, phase detection sensitivity decreases. The reasons for this may be explained as follows. The phase of a symbol changing point cannot be determined due to inter-symbol interference caused by a wavelength dispersion correction error in the compensation of waveform distortion, or the H axis component (horizontally polarized component) and a V axis component (vertically polarized component) contained in the output of the phase detector PD cancel each other because the positive sign and the negative sign are reversed between these components. If the sensitivity is lowered, the phase following capability is degraded and dephasing (out-of-phase synchronization) is likely to occur.
To compensate for the low sensitivity, the phase detector 1512 of a selection-diversity combining type illustrated in FIG. 1B is proposed. In the phase detector 1512 of the selection-diversity combining type, multiple equalization filters 281-1 through 281-N with different equalization characteristics are placed before sensitivity monitoring phase detectors 282-1 through 282-N. By using the multiple processing lines with different characteristics of equalization, the phase following capability can be maintained, even if a phase detection result is not acquired at a certain phase detector, based upon the outputs of the other phase detectors. In particular in FIG. 1B, only the phase detection signals having sensitivity monitoring levels over a threshold value are summed up at the combining part 313 to generate a phase detection signal. Consequently, an SN ratio is improved. Prior to combining the phase detection results, sensitivity correction coefficients are generated based upon the sensitivity monitoring values (at 321-1 through 321-N) and multiplied with the associated phase signals.
However, even though selection-diversity combining is employed using only those phase detection values with high sensitivities, influence of fluctuation of polarization may hardly be avoided. Therefore, there is a demand for a digital coherent receiver and a phase control method that can maintain stable control on the sampling phases regardless of polarization mode dispersion or polarization fluctuation.